1. FIELD OF THE INVENTION
The present invention relates to a heat regenerator which indirectly utilizes the hot gases exhausted by a furnace or engine to heat the air supply to it, through the use of heat transfer fluids which function as an efficient heat transfer medium due to implementation of the counterflow and direct contact heat transfer principles.
2. BACKGROUND OF THE PRESENT INVENTION
The idea of using indirect heat exchange between the exhaust gases and incoming air of a furnace through the medium of a heat transfer fluid has been previously proposed, as is representatively evidenced by U.S. Pat. Nos. 2,609,799; 2,635,587, and 2,681,047. However, in actual practice, these devices of the prior art have manifested some serious shortcomings due to their serious design limitations to which the present invention addresses itself. These air preheaters of the prior art are generally of the surface recuperative type, or of the regenerative rotary metallic-matrix type, which imposes rather severe restrictions on the general design of the furnance, as the hot exhaust gases duct, or flue, and the air intake duct must be brought to the same basic locality of the air preheater apparatus. Also, neither of these types of air preheaters can be controlled to regulate the rate or degree of heat transfer effected by the apparatus. The surface-type recuperators of the indirect type air heater of the prior art circulate the same fluid (i.e. the heat transfer medium) through coils disposed in both the exhaust gases and air intake ducts. This approach requires costly coils having large surface areas. Further, these recuperators do not utilize the counterflow principle of heat exchange, which would result in optimum heat exchange, and are therefore inefficient. Yet further, due to their inherent disability or difficulty to regulate the heat exchange process, these air preheaters have a tendency to undercool to temperatures below the dew-point temperature of the hot exhaust gases, thereby causing their metallic surfaces to be attacked by the condensing moisture, ultimately resulting in irreversible corrosion thereof, thereby necessitating replacement of the coils, which is, of course, very costly.
The present invention eliminates these and other shortcomings and disadvantages of these air preheaters of the prior art.
The present invention teaches the use of one or more heat transfer fluids circulated through one or more regenerative loops to effect indirect counterflow heat exchange between the hot gases leaving the furnace or engine and the cold air introduced to said furnace or engine to facilitate the combustion process which takes place therein. The regenerative type heat exchanger of the present invention obviates the necessity of bringing the exhaust gases duct and the intake air duct to the same locality, thereby allowing the designer complete freedom in laying out the ducts. Each of said regenerative loops comprises two recuperators and all of the necessary connecting conduits; circulating pumps or blowers; reservoirs; filters; and fittings. One of the two recuperators of each said loop is disposed in the exhaust gases duct and the other is disposed in the incoming air duct. In operation, the apparatus of the present invention basically works in the following manner:
The heat transfer fluid is circulated by means of said pumps or blowers through each said regenerative loop, thereby cooling the hot gases being exhausted through the exhaust flue of said furnace and simultaneously heating said heat transfer fluid; the now hot heat transfer fluid is then recooled by the incoming air stream, thereby simultaneously heating the incoming air stream--said heat transfer fluid is thus continuously circulated in a loop exchanging heat between the hot exhaust gases and the cold incoming air. Further, said regenerative loops are arranged in such a manner relative to each other in the two ducts so as to implement the counterflow principle of heat exchange between the exhaust gases and the incoming air. This is effected by placing said regenerative loops successively in the two ducts according to the temperature gradients in said ducts; therefore, the highest temperature loop would convey heat from the hottest gases to the hottest air, while the loop of lowest temperature conveys heat from the coolest gases to the coldest air, thereby optimizing the heat exchange process, in a manner heretofore unknown. The present invention provides, except for the loops exposed to very high temperatures, preferably, recuperators of the open type, wherein the heat transfer fluid (i.e. the heat exchange medium) is introduced into direct contact with the exhaust gas stream and the incoming air stream. Said heat transfer fluid would be jetted, sprayed, splashed, or introduced in any other suitable manner. Therefore, due to the extended interface between the heat transfer fluid and the gas and the heat transfer fluid and the air, each recuperator would be very compact, as less volume need be provided, relative to the surface-type regenerators heretofore known. After the heat transfer fluid is sprayed into the exhaust gas or incoming air stream, it is collected in a trough disposed at the bottom of each duct below each "spraying point." Just downstream of each point where said heat transfer fluid is sprayed into the gas or air stream, a suitable separator is disposed vertically across said exhaust gases duct or said incoming air duct, to effect the removal of all liquid droplets which remain suspended in the gas stream or air stream therefrom into said trough disposed beneath each recuperator at the point at which said heat transfer fluid is sprayed into the gas or air stream. The heat transfer fluid thus collected within said troughs is then delivered by pump or gravity to the other recuperator of the same loop. Said suitable separator can be an array of Z-shaped baffles, for example.
The heat transfer fluid to be used in the open, or direct-contact recuperator, such as is advocated by the present invention, should have a very low vapor pressure at its maximum operating temperature to minimize evaporation, thereby minimizing the amount of make-up fluid that need be added to replenish the supply of said heat transfer fluid. For safety purposes, the flash point of the heat transfer fluid--if it has one--should be well above the maximum operating temperature of the recuperator. Further, the vapors of such a heat transfer medium should be non-toxic in order to avoid polluting the exhaust gases released to the atmosphere. Fluids suitable for use in said regenerative loops with open-type recuperators are available with flash points up to approximately 290.degree. C. (550.degree. F.), for example, polyphenyl ethers. It should be noted, however, that closed or surface-type recuperators are utilized by the present invention, when necessary, especially in the loops exposed to excessively high temperatures. For these closed loops, the above restrictive conditions could be greatly relaxed: Indeed, a gas, vapor, salt, or liquid metal could be used if necessary. All recuperators of the present invention, whether of the open or surface type, could be made with any number of passes, wherein each said pass would comprise any numbers of banks, and wherein each said bank comprises application means for introducing said heat transfer fluid into the gas or air stream (e.g. an orifice having a spray nozzle mated in fluid communicating relationship thereto). Another major advantage of the present invention over the prior art is the simplicity with which it can be controlled to regulate the rate or degree of heat transfer effected by the apparatus. Some simple methods of controlling the flow rate of the heat transfer fluid (i.e. to control the heat exchange process) include the following:
a. shutting off some of said passes or recuperator banks;
b. varying said pump or blower speed, thereby regulating the flow rate at which said heat transfer fluids are circulated through said regenerative loops;
c. cutting off or throttling the circulation in one or more of said regenerative loops.
Therefore, since the rate of heat exchange varies proportionately with the flow rate of the heat transfer fluid being circulated through said regenerative loops, incidences wherein said regenerative loops will undercool to temperatures below the dew-point temperature of the hot exhaust gases, can thereby be substantially eliminated, thereby substantially preventing the possibility of the metallic surfaces of the loops being attacked by the condensing moisture, thereby ultimately preventing corrosion thereof. This precautionary feature, therefore, thereby virtually eliminates the necessity of replacing any parts of the present invention due to corrosion. An additional advantage of the present invention is that the heat transfer fluid may dissolve some of the gaseous pollutants and/or wash down any solid particles that may be held in suspension by the exhaust gases. The recuperators would thus act as washers or scrubbers of the exhaust gases and, therefore, could replace at least some parts of the pollution or emission-control equipment. It should be noted that filters would be provided in the loop through which the heat transfer fluid and the dissolved gaseous pollutants and the suspended solid particles would be conveyed, to thereby eliminate them from said heat transfer fluid.
Many other objects and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.